VIENNA — Iran and a group of six nations led by the United States have agreed to a historic accord to significantly limit Tehran’s nuclear ability for more than a decade in return for lifting international oil and financial sanctions against Iran, a senior Western diplomat involved in the negotiations said on Tuesday.

The deal culminates 20 months of negotiations on a deal that President Obama had long sought as the biggest diplomatic achievement of his presidency.

A formal announcement of the agreement was expected later on Tuesday, when foreign ministers from Iran and the six nations it has been negotiating with will meet at a United Nations complex here in Vienna. Catherine Ray, a spokeswoman for the European Union, said a final plenary meeting of the six nations — Britain, China, France, Germany, Russia and the United States — will take place at 10:30 a.m. in Vienna, followed by a news conference, but she provided no further details.

"What Julia Child did for French cooking, Pundita is doing for foreign policy discussion. She's opened a haute pursuit to ordinary people." - Caesar Rodriguez

"Few bloggers think as deeply as Pundita does. And few have her foresight." - Uppity Woman

"There is no other blogger like Pundita. She has carved a niche - part history and part humint - that makes any essay of hers a thing unto itself. In addition there are the sparkles of her dry wit sprinkled through her work." - Dymphna, Gates of Vienna